Modern televisions with LED backlights have tended towards longer strings of serially connected LEDs that are controlled as one circuit. These strings, containing from 10 to 40 LEDs, can require voltages between 30 and 150 volts in order to operate effectively. In such strings, the current is regulated in order to control the color of the individual LEDs—more current tends to yield a cooler color of white, while less current tends to yield a warmer color. In order to control the color spectrum of the output, the current is held constant while the brightness is controlled using a PWM on/off mechanism—more time on yields a brighter screen.
The current through these strings is typically controlled by a current sink connected from the cathode of the LED string to ground. The current sink is cycled on and off at a given PWM duty cycle by a controller within the television, resulting in a brighter or darker screen. Such a basic circuit is shown in FIG. 1.
A problem arises because variation in the LED manufacturing process renders the forward voltage across the LEDs that compose the string to be somewhat random, and as a result, the forward voltage across “the entire string” is random as well. In the same FIG. 1, a variable forward voltage across the LEDs results in a variable voltage across the current sink—a larger string voltage results in a smaller voltage across the voltage sink, and vice versa. This occurs because in a typical application, the cathode of the current sink is at a fixed voltage, often ground. If a typical current sink requires 1.0 volts to operate, the excess voltage across the current sink, from the “floating” anode to the “fixed” cathode, can range from 1.0 volts to 5 times that or more, causing the current sink to dissipate all of the excess power. Excess power dissipation creates heat on the PCB containing the LED driver circuitry which reduces electronic component life times.